Contact lenses, which are to be manufactured economically in large unit numbers, are preferably manufactured by the so-called mold or full-mold process. Lens molds for making contact lenses are well known to a person skilled in the art and, for example, are employed in cast molding or spin casting. For example, a mold (for full cast molding) generally comprises at least two mold sections (or portions) or mold halves, i.e. first and second mold halves. The first mold half defines a first molding (or optical) surface and the second mold half defines a second molding (or optical) surface. The first and second mold halves are configured to receive each other such that a lens forming cavity is formed between the first molding surface and the second molding surface. The molding surface of a mold half is the cavity-forming surface of the mold and in direct contact with a fluid polymerizable composition.
Most common mold material used for casting molding contact lenses is either a polypropylene or polystyrene. However, various problems have been associated with injection molding of such materials. For example, many grades of polypropylene exhibit poor processibility; while others, although having excellent processibility, result in an unsatisfactory mold due to surface distortions and the like. Several patents or patent application publications described methods to identify or improve polypropylene properties to improve mold quality. U.S. Pat. No. 5,843,346 discloses that improvements in the static cast molding of contact lenses. The improvement comprises injection molding at least one of the mold sections from a thermoplastic polyolefin resin having a melt flow rate of at least about 21 g/10 minutes. US 2006/0051454 A1 also discloses the improvements in the static cast molding of contact lenses. The improvement comprises injection molding at least one of the mold sections from a Ziegler-Natta catalyst based polyolefin having a melt flow rate of less than 21 g/10 min. U.S. Pat. No. 6,582,631 discloses at least one of the male and female mold sections is injection molded from a metallocene-catalyzed thermoplastic polyolefin resin.
However, there still exists a need in the art for an improved method of cast molding contact lenses with molds formed from polypropylene with improved rheology properties and at the meantime provide consistently superior qualities in the molded lenses.